(CNN) -- You think of yourself as a reasonably hip, attractive person. You wear the latest styles and present your best side to the world. But then you open your passport and there it is on Page 2: A blast from the past staring back at you.

Maybe you were just recovering from a tragic haircut when that fateful picture was taken. Perhaps you were at the height of your facial hair experiment phase. Maybe the Goth look seemed like the coolest thing ever at the time.

Now, it's an image that makes your friends giggle. Worse, you fear it could force Border Patrol agents to do a double-take at the airport when they see the real you.

So what happens when you don't resemble your passport photo anymore?

"I feel like if I go through customs, they're maybe going to look at me and be like, 'Are we sure this is the same person?' Because I do look drastically different," said iReporter Brian Wilson, a student at Miami University in Oxford, Ohio.

His passport photo -- taken in 2007 when he was a college freshman -- shows a mass of curls surrounding his face à la Robert Plant. But Wilson, 23, now sports very short hair.

Sometimes, the changes can reflect a serious transformation in a traveler's life.

iReporter Kayla Strauss weighed about 400 pounds when her passport photo was taken four years ago. She's lost 240 pounds since then, prompting comments from Border Patrol agents when she recently returned from a trip to Japan.

"Obviously, from my facial structure they can still tell it's me. But they say, 'Oh, you've lost a lot of weight, what happened? How did you do it?' " said Strauss, 22, who lives in St. Louis.

"When I look at the photo, I feel proud that I lost the weight, but I also feel like I'm a new person."

And then there are those photos from long ago that you wish you could just forget.

"I personally hate the picture and I am embarrassed to use it," said iReporter Ryan Fleeman, whose passport photo was taken in 2004 and shows a much more boyish face, complete with messy bangs and glasses.

A friend who recently saw the picture for the first time told him he looked like a serial killer and "laughed hysterically," said Fleeman, 25, who lives in Alexandria, Virginia.

You can see more fun examples of passport photos and what their owners look like now in an online project, titled "Passport and Reality."

Significant changes

So, should you run to the passport photo place and fill out a new application when your picture starts to look a bit stale?

In most cases, the answer is no.

"As passports are valid for 10 years, the Department (of State) understands that applicants may undergo significant changes between the time the passport is initially issued and the time it is renewed," said Rebecca Dodds, a spokeswoman for the Bureau of Consular Affairs.

If your passport has not yet expired, new photos would only be required if your appearance has "significantly changed" from the face depicted in your photo, according to the Department of State website.

Growing a beard or coloring your hair is not considered a significant change in the government's eyes. Officials who deal with passports are trained to focus on facial features, Dodds said, so a new hairstyle wouldn't be a distraction.

Agents look for basic facial characteristics that never change, echoed Joanne Ferreira, a spokeswoman for U.S. Customs and Border Protection, but the agency did not respond to further questions about the process.

So when do you have to get a new photo?

Undergoing significant facial surgery or trauma would be one example, the State Department says on its website. Changing your gender, or adding or removing many facial piercings or tattoos would be another. Losing or gaining lots of weight may also fall into the "significant change" category.

Bottom line: If you cannot be identified from your current passport photo, you should apply for a new passport, even if the old one has not expired, Dodds said. (That also means paying the passport renewal fee.)

'Bug-eyed look of terror'

No one is really pleased with their passport photos, of course. Visit a few travel message boards and you soon realize the universal dread the pictures inspire.

"I look tired, drunk and stoned in my passport photo, which is usually how I look after a long, nasty flight," one poster wrote on Fodors.com.

Another fumed that the photographer kept yelling at her to look at the camera "and then took this photo of me with a bug-eyed look of terror!!"

Still, U.S. citizens have a bit more opportunity to look friendly in their passports. A smile is acceptable in a U.S. passport photo, unless it is extremely exaggerated or it obscures your facial features, Dodds said. (So no squinting.)

Maybe that's why one in six Britons say that showing their passport photo to a colleague would be more embarrassing than falling over in public, spilling food on themselves or discovering that their flies were open, according to an Ipsos survey of 5,000 Europeans.

When it does come time to get a new passport photo, the U.S. government provides lots of dos and don'ts on its website, complete with somewhat humorous examples modeled by Department of State employees.

Each year, the department returns just over 1% of all photos to passport applicants, Dodds said. The most common reasons are blurry or damaged photos, or photos that don't have a plain, off-white background.